Various adhesive products are commercially available for asserted use as sealants or adhesives to repair or patch polymeric elastomeric materials such as natural rubber, plasticized polyvinyl chloride, polychloroprene and the like. Such materials are typically used in a variety of outdoor recreation articles, such as tents, rubber boots, waders, inflatable objects (e.g., rafts, inner tubes), etc. These articles are used in a variety of environmental conditions, including high humidity and rain, and are often subject to abrasion or other potentially deleterious conditions.
Many of the commercially available adhesive compositions for use with such materials are high-solvent content polyurethanes, polyvinyl chloride, or rubber blends. Such adhesives are, however, not individually suitable for use with a wide variety of polymeric substrates. Some, for example, have extended cure times which means a repaired item may not be available for use for many hours or even days. When exposed to environmental extremes or high moisture conditions, some form only weak adhesive bonds to certain substrates and others fail to form any bond at all. Additionally, some of these products contain solvents which can cause detrimental foaming or bubbling while curing, and which present certain health and environmental hazards. When the product is a two-part adhesive system, formation of useful bonds may require close measurement, which may be problematic for the user under various environmental and field conditions. Further, some, like conventional rubber cement, may require a separate patch to be useful. For good bonding to rubber, vulcanization is typically required to achieve a satisfactory bond.
It is therefore desirable to provide an adhesive composition capable of curing at ambient temperatures; eliminating the necessity for solvents and the problems associated therewith; and forming useful bonds in about one hour and strong bonds in 24 hours or less.
In addition, when a preferred excess of NCO stoichiometry is utilized, the composition is capable of adhering strongly to a variety of polymeric/elastomeric substrates, even in the presence of moisture and environmental extremes; forming useful bonds despite inaccurate component measurements, if required; curing in contact with, or even submerged in, water; and forming a high strength flexible elastomer itself upon curing.
Polyurethanes have many practical applications as foams, elastomers, adhesives, coatings and sealants. Polyurethane adhesives typically involve the catalyzed reaction of hydroxyl-terminated oligomers (i.e., polyols) and di- or polyisocyanates. However, exposure of polyurethane adhesive compositions to water or moisture conditions often causes an unsatisfactory reduction in adhesive bond strength. Additionally, catalysts for the polyol/isocyanate reaction often also catalyze a water/isocyanate reaction, thus causing bubbling and foaming when used in wet or high humidity conditions or when applied to wet substrates.
Polyureas are obtained by the polyaddition of substantially equivalent amounts of polyisocyanates and polyamines (especially diamines) having terminal amino groups, typically resulting in rigid compositions upon curing. Such materials are typically used for injection molding of automotive parts, where fast cure and short cycle times are desired, and for molding rigid articles, such as skate wheels and ski boots. However, polyureas generally are not suitable for formulating room temperature curable liquid compositions.
Polyurethaneureas are formed by first preparing an isocyanate-terminated prepolymer from a macrodiol, such as a polyester diol or a polyetherdiol, using excess polyisocyanate, resulting in a prepolymer having isocyanate end groups. This prepolymer is in turn reacted with polyamines to yield polyurethaneureas. Such materials are characterized by alternating mobile soft segments, which provide flexibility, and hard segments, which contribute to cohesive strength and sometimes to adhesive strength.
U.S. Pat. No. 4,663,201 discloses a method of coating an object with a polyurea resulting from the reaction of a polyisocyanate having at least three isocyanate moieties with from about 0.8 to 1.2 equivalents of a sterically hindered secondary aromatic diamine.
U.S. Pat. No. 3,817,940 discloses a synthetic polymer produced by the polyaddition reaction of about 0.9 to about 1.2 equivalents of polyisocyanate per equivalent of an ortho-substituted aminobenzoic acid ester or amide, i.e., a diamine. Such diamines are solid at room temperature, and thus would be very difficult to use in field conditions.
U.S. Pat. No. 4,345,058 discloses a urethane prepolymer repair system provided by an isocyanate-terminated prepolymer of polytetramethylene glycol in a solvent. It is believed this material is sold commercially as "Aquaseal." This product cures slowly over a 48 hour period, so it is not highly useful for field use or quick repair. Bubbles can also form due to the solvent or to carbon dioxide formed while the material is still soft. After final cure, the material exhibits good adhesion and abrasion resistance.
U.S. Pat. No. 4,578,446 and U.S. Pat. No. 4,714,512 disclose polyurethanes made from isocyanate-terminated prepolymers and sterically hindered liquid aromatic diamines. It is very difficult to formulate repair materials with a convenient 1:1 mix ratio from these materials. These aromatic diamines are not oligomeric and also tend to be very dark in color.
U.S. Pat. No. 4,328,322 discloses a polyurea polymer formed from the reaction of essentially equivalent amounts of low molecular weight polyisocyanates and oligomers terminated with p-aminobenzoate. The use of isocyanate prepolymers is not disclosed nor is the use of excess equivalents of isocyanate to amine, which allows for formation of useful bonds despite inaccurate measurements. In the working examples, a 4:1 mix ratio is described, which results in high tensile strength polyureas.
By use of polyisocyanate-terminated prepolymers and/or a liquid polyisocyanate, combined with oligomeric amines, the resulting cured polymers of the invention exhibit improved flexibility and adhesion to polymeric or rubbery substrates, fulfilling the needs discussed previously.